Color corrected projection lenses employing diffractive optical surfaces

ABSTRACT

A projection television system ( 10 ) is provided which has a CRT ( 16 ) and a projection lens system ( 13 ) for forming an image on a screen ( 14 ). The projection lens system ( 13 ) is characterized by a diffractive optical surface (DOS) which provides color correction for the lens system. The diffractive optical surface (DOS) can be formed as part of a diffractive optical element (DOE) or as part of an existing lens element of the lens system. The diffractive optical surface (DOS) is located between the object side (S 2 ) of the lens&#39; first lens unit (U 1 ) and the image side (S 11 ) of the lens&#39; third lens unit (U 3 ). The distance between the diffractive optical surface (DOS) and the lens&#39; aperture stop (AS) is less than 0.1·f 0 , where f 0  is the focal length of the projection lens ( 13 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase under 35 USC §371 ofInternational Application No. PCT/US99/26645, filed Nov. 12, 1999, whichwas published in English under PCT Article 21(2) on May 18, 2000 asInternational Publication No. WO 00/28353. This application claims thebenefit under 35 USC §119(e) of U.S. Provisional Application No.60/108,143 filed Nov. 12, 1998, the contents of which in its entirety ishereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to projection lens systems for use in projectiontelevisions and, in particular, to color corrected, wide field of view,high numerical aperture projection lens systems for use with cathode raytubes (CRTs), including cathode ray tubes having curved faceplates.

BACKGROUND OF THE INVENTION

Various color-corrected high image quality lenses for use in highdefinition TV displays (HDTV) and in the projection of data and graphicsare known in the art. These lenses are most frequently used in “frontscreen” two piece systems, i.e. systems where the projector and thescreen are two different units. As a result of the long distance betweenthe projector and the screen, most of the lenses used in such systemshave a half field of view of under 30°.

In recent years, one piece projection TVs have become increasinglypopular. These systems utilize a “rear screen” configuration in whichthe image is projected onto the rear surface of a translucent screenwhich is combined with the projector into a single unit. To achieve asmall overall size for such systems, the lens must have a field of viewas wide as possible.

To help achieve this goal and to provide for an increased amount oflight at the outer portions of the image, CRTs having curved faceplatesare most often used in this application. The faceplates of such CRTs areplano-convex shaped with the phosphor being deposited onto the curvedside of the faceplate. As a result, the outer portion of the phosphorside of the faceplate curves towards the lens.

Presenting the CRT image on a surface concave towards the projectionlens allows the lens to achieve a half field of view in excess of 40°.However the control of electron beam spot size on a curved phosphorsurface is much more difficult than on a flat surface. Spot size controlis important since a small and well controlled spot size is required toproduce a high quality image.

As long as spot size was fairly large, projection lenses did not need tobe corrected for axial color. However, since the introduction of digitalTV (e.g., satellite TV and DVD), the quality level of one piece rearprojection TV sets for consumer use has been significantly raised.

Manufacturers of such systems are now more willing to use morecomplicated electronics to minimize and control the size of the spot ona curved phosphor surface, e.g., they are willing to produce spot sizeswhose sizes are 0.15 millimeters or less. Consequently, new high qualitywide field of view large aperture lenses are needed to compliment thehigher quality outputs of curved phosphor CRTs. As with the optics usedin data and graphics projection TV systems, these new lenses need to becorrected for color.

A typical color corrected lens used with a flat faceplate CRT consistsfrom long conjugate to short of a front weak aspherical unit, a mainpower unit which includes a color correcting doublet and a strongpositive element having most of the power of the lens, a corrector unitfollowing the main power unit and having at least one asphericalsurface, and a strong negative power unit associated with the CRTfaceplate and providing most of the correction for the field curvatureof the lens. See Kreitzer, U.S. Pat. No. 4,900,139.

From the image side, the main power unit typically has a negativeelement followed by a positive element of similar focal length but ofopposite sign. These two elements provide color correction for the lensand their combined shape is typically meniscus towards the longconjugate. The single positive element providing most of the power ofthe lens usually follows the color correcting doublet.

Moskovich, U.S. Reissue Pat. No. 35,310, discloses color correctedprojection lenses having three lens units wherein each of the first andsecond units has a positive low dispersion element and a negative highdispersion element.

Co-pending and commonly assigned U.S. patent application No. 09/005,916,filed Jan. 12, 1998, in the name of Jacob Moskovich and entitled “ColorCorrected Projection Lenses For Use With Curved Faceplate Cathode RayTubes,” discloses projection lenses for use with curved CRTs wherein thesecond lens unit has two positive lens elements at least one of which isat the image side of the lens unit.

The foregoing approaches to achieve color correction have each employedat least one negative lens element of high dispersion which has meantthat additional positive power had to be added to the system tocompensate for the negative power of the negative element. Theadditional positive power has taken the form of stronger positiveelements or, in many cases, the inclusion of an additional positiveelement in the system. The incorporation of additional positive andnegative elements has increased the cost, complexity, and weight of thelens system. In particular, weight has been increased when the colorcorrection has been achieved using glass elements. The use of glasselements has also meant working with flint glass for the negative highdispersion elements. As known in the art, flint glass is more difficultto work with than crown glass.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a projection lens system which (1) has a large aperture, i.e., af/number of about 1.2 or less, (2) has a wide field of view, i.e., ahalf field of view of at least 35°, (3) provides a high level ofcorrection of both chromatic and monochromatic aberrations when usedwith cathode ray tubes, including cathode ray tubes having curvedfaceplates, and (4) achieves chromatic aberration correction with aminimum of additional lens elements and in some cases no additional lenselements.

To achieve these and other objects, the invention provides a projectionlens system which from long conjugate to short comprises:

(A) a front lens unit (first lens unit; U1) comprising at least oneaspherical element (i.e., an element having at least one asphericalsurface), said front lens unit having a short conjugate side (S2 inTables 1 and 2),

(B) a positive power lens unit (second lens unit; U2) which preferablyprovides the majority of the power of the lens system,

(C) a corrector lens unit (U_(CR)) comprising at least one asphericalelement (i.e., an element having at least one aspherical surface), and

(D) a strong negative power unit (third lens unit; U3) associated withthe CRT faceplate having a strong concave surface (S11 in Tables 1 and2) facing the long conjugate and providing most of the correction of thefield curvature of the lens, said strong negative power unit having along conjugate side (S11 in Tables 1 and 2),

wherein the lens system includes at least one diffractive opticalsurface (DOS) which at least partially corrects the axial color of thelens system and which is located between the short conjugate side of thefront lens unit and the long conjugate side of the strong negative powerunit.

The diffractive optical surface will in general have positive opticalpower. Accordingly, unlike the use of a high dispersion negative lenselement to achieve color correction, the use of a diffractive opticalsurface does not require the incorporation of additional positive powerinto the system to balance added negative power. Indeed, the use of apositive diffractive optical surface can allow for at least somereduction in the power of one or more positive elements in the systemwhich, in turn, can facilitate the overall correction of the system'saberrations.

The at least one diffractive optical surface can be a blazed kinoform ora binary approximation to a blazed kinoform and can comprise (1) asurface of a separate optical element (e.g., a diffractive opticalelement (DOE) which is plano on one side and has a diffractive opticalsurface on the other), or (2) a surface of an element which forms partor all of the positive power lens unit (U2) or the corrector lens unit(U_(CR)).

When formed as part of the positive power lens unit or the correctorunit, the diffractive optical surface provides color correction to thelens system without the need for any additional lens elements. Whenformed as a surface of a DOE, only one element is required. Accordingly,in either case, the diffractive optical surface of the invention is ableto provide color correction for a projection lens system with a minimumincrease in the system's complexity, cost, and weight. Although lesspreferred, multiple diffractive optical surfaces can be used if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-2 are schematic side views of lens systems constructed inaccordance with the invention.

FIG. 3 is a schematic side view of a lens system having a comparableconstruction to the lens systems of FIGS. 1 and 2, but without adiffractive optical surface.

FIGS. 4A, 4B, and 4C are calculated plots of lateral aberration versusrelative entrance pupil coordinates for the lenses of FIGS. 1, 2, and 3,respectively, for an image to object magnification of −0.117. Theparameters for these figures appear in Table 4.

FIGS. 5A, 5B, and 5C are calculated plots of lateral aberration versusrelative entrance pupil coordinates for the lenses of FIGS. 1, 2, and 3,respectively, for an image to object magnification of −0.101. Theparameters for these figures appear in Table 5.

In FIGS. 4 and 5, solid lines represent TAN data, dashed lines representSAG data, and dotted lines represent SAG-Y data. The wavelengths for thecircle, triangle, and square data points are 0.546 microns, 0.1480microns, and 0.1644 microns, respectively. The H′ dimensions given inthese figures are in millimeters and the vertical scale is in units of0.1 millimeters.

FIG. 6 is a schematic diagram of a rear projection TV employing a lenssystem constructed in accordance with the invention.

The foregoing drawings, which are incorporated in and constitute part ofthe specification, illustrate preferred embodiments of the invention,and together with the description, serve to explain the principles ofthe invention. It is to be understood, of course, that both the drawingsand the description are explanatory only and are not restrictive of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lens systems of the invention preferably include a first lens unit,a second lens unit, a third lens unit, and a corrector lens unitwherein: 1) the first lens unit includes at least one asphericalsurface; 2) the second lens unit has a strong positive optical power; 3)the third lens unit corrects for the field curvature of the lens systemand has a relatively strong negative optical power; and 4) the correctorlens unit provides correction for, among other things, aberrations dueto off-axis rays and has a relatively weak optical power. The systemsalso include at least one diffractive optical surface for providing atleast partial color correction to the lens system.

The first lens unit serves to correct aperture type aberrations,including spherical aberration and coma, and can be composed of one ormore lens elements. Preferably, the element or elements of this unit areformed from plastic materials, e.g., acrylic plastics.

The second lens unit preferably provides the majority of the lenssystem's positive optical power. Although this unit can include multiplelens elements and can have one or more aspherical surfaces, preferablythe unit consists of a single glass element having spherical surfaces.

The corrector unit and third lens unit serve to correct off-axisaperture dependent aberrations and field dependent aberrations,respectively. In particular, the corrector unit is effective in dealingwith oblique spherical aberrations, while the third lens unit iseffective in reducing the system's field curvature.

The corrector lens unit can be composed of one or more lens elements.Preferably, the element or elements of this unit are composed of plasticmaterials.

The third lens unit is preferably composed of an aspherical plastic lenselement which contacts the fluid which couples the lens system to thefaceplate of the CRT. If desired, the aspherical plastic lens element ofthe third lens unit can include an absorptive color filter material inaccordance with Wessling, U.S. Pat. No. 5,055,922.

Quantitatively, the ratio of the absolute value of the focal length (f₁)of the first lens unit to the overall focal length (f₀) of theprojection lens is preferably greater than 2.5; the ratio of the focallength (f₂) of the second lens unit to the overall focal length of theprojection lens is preferably less than 1.5; the ratio of the absolutevalue of the focal length (f_(CR)) of the corrector lens unit to theoverall focal length of the projection lens is preferably greater than2.0; and the ratio of the absolute value of the focal length (f₃) of thethird lens unit to the overall focal length of the projection lens ispreferably less than 2.5.

The diffractive optical surface (DOS) provides at least partial axialcolor correction for the projection lens. To design a projection lensemploying a DOS, the Sweatt model can be used wherein the diffractivesurface is treated as a refractive surface having a very large index ofrefraction (typically 9999) and a V-number of, for example, ₃₁ 3.4 forlenses which are to be used in the 0.1486 to 0.1656 micron range. See W.C. Sweatt, “Mathematical Equivalence between a Holographic OpticalElement and an Ultra High Index Lens,” Journal of the Optical Society ofAmerica, 69:486-487, 1979.

The first order theory of thin lens achromatic doublets is used tocalculate the diffractive power required for achromatization. See, forexample, Warren J. Smith, Modern Optical Engineering, Second Edition,McGraw-Hill, Inc., New York, N.Y., 1990, pages 372-375.

This theory gives the following relationship between the optical powerΦ_(DOS) of the diffractive optical surface, the optical power Φ_(L) ofthe rest of the lens, and V_(L) and V_(DOS), the Abbe numbers of theaverage lens glass or plastic (typically about 60) and the diffractiveelement (e.g., −3.4), respectively:

Φ_(DOS)/Φ_(L) =−V _(DOS) /V _(L)

For the lenses of FIGS. 1 and 2, the total optical power which thelenses were designed to provide was approximately 0.1014 mm⁻¹.Application of the above formula then gave a value of about 0.10007 mm⁻¹for the power of the diffractive element required to achieve total colorcorrection. Assuming a convex-plano DOE and using a refractive index of9999, a curvature “c” of about 0.00000007 mm⁻¹ for the convex surfacewas obtained using the relationship:

Φ=(n−1)c

where “n” is the index of refraction of the DOE.

In the lenses of FIGS. 1 and 2, an optical power of 0.0005 mm⁻¹ for theDOE, rather than 0.0007 mm⁻¹, was in fact used. This results in aprojection lens which is not totally color corrected but has the benefitof improving the diffractive efficiency of the DOS.

As illustrated in FIGS. 1 and 2, the diffractive optical surface islocated between the object side of the first lens unit and the imageside of the third lens unit, i.e., between surfaces S2 and S11 in thesefigures. Preferably, the DOS is located in the vicinity of theprojection lens' aperture stop (AS). In particular, for a projectionlens having a focal length f₀ and a distance “d” between the DOS and theaperture stop, the ratio d/f₀ is preferably less than 0.1 and mostpreferably less than 0.05. Increasing the d/f₀ ratio above 0.1 isundesirable since it leads to unacceptably high levels of lateral color.For the projection lenses of FIGS. 1 and 2, this ratio is approximately0.01.

The DOS can be made using a variety of techniques now known orsubsequently developed. Examples of such techniques including machiningof individual elements using, for example, a diamond turning machine or,more preferably, producing a master mold and forming elements having thedesired diffractive surface using injection molding techniques. Binaryapproximations to a DOS surface can be produced using photolithographytechniques known in the art. Elements having diffractive opticalsurfaces, especially when made by molding, will generally be composed ofa plastic material, e.g., an acrylic polymer, although other materials,e.g., glass materials, can be used if desired.

FIGS. 1-2 illustrate various projection lenses constructed in accordancewith the invention. FIG. 3 shows a projection lens having a comparableconstruction to the lens systems of FIGS. 1 and 2, but without adiffractive optical surface. Corresponding prescriptions appear inTables 1-3. HOYA or SCHOTT designations are used for the glassesemployed in the lens systems. Equivalent glasses made by othermanufacturers can be used in the practice of the invention. Industryacceptable materials are used for the plastic elements.

The aspheric coefficients set forth in the tables are for use in thefollowing equation:$z = {\frac{{cy}^{2}}{1 + \lbrack {1 - {( {1 + k} )c^{2}y^{2}}} \rbrack^{1/2}} + {Dy}^{4} + {Ey}^{6} + {Fy}^{8} + {Gy}^{10} + {Hy}^{12} + {Iy}^{14}}$

where z is the surface sag at a distance y from the optical axis of thesystem, c is the curvature of the lens at the optical axis, and k is aconic constant, which is zero for the prescriptions of Tables 1-3.

The designation “a” associated with various surfaces in the tablesrepresents an aspheric surface, i.e., a surface for which at least oneof D, E, F, G, H, or I in the above equation is not zero. All dimensionsgiven in the tables are in millimeters. Tables 1-3 are constructed onthe assumption that light travels from left to right in the figures. Inactual practice, the viewing screen will be on the left and the CRT willbe on the right, and light will travel from right to left.

The CRT faceplate constitutes surfaces 13-14 in Tables 1-2 and surfaces11-12 in Table 3. A coupling fluid is located between surfaces 12-13 inTables 1-2 and surfaces 10-11 in Table 3. The material designations forthese components are set forth as six digit numbers in the tables, wherea N_(e) value for the material is obtained by adding 1,000 to the firstthree digits of the designation, and a V_(e) value is obtained from thelast three digits by placing a decimal point before the last digit. Theasterisks in Tables 1 and 2 represent the index of refraction and theAbbe numbers used in the Sweatt model for the DOS, i.e., a N_(e) valueof 9999 and a V_(e) value of −3.4.

In Table 1, the first lens unit comprises surfaces 1-2, the second lensunit comprises surfaces 4-5, the DOE comprises surfaces 6-8, thecorrector lens unit comprises surfaces 9-10, and the third lens unitcomprises surfaces 11-14. Surface 3 is an optional vignetting aperture.

In Table 2, the first lens unit comprises surfaces 1-2, the second lensunit comprises surfaces 3-4, the DOE comprises surfaces 5-7, thecorrector lens unit comprises surfaces 9-10, and the third lens unitcomprises surfaces 11-14. Surface 8 is an optional vignetting aperture.

Table 6 summarizes various properties of the lens systems of theinvention. As shown therein, the lens systems of Tables 1-2 have thevarious preferred properties referred to above. In this table, thedesignation “½w” represents the half field of view of the lens system.

FIGS. 4 and 5 compare the chromatic aberration of the lenses of FIGS. 1and 2 which employ the invention with the chromatic aberration of thelens of FIG. 3 which has a comparable construction but without a DOS. Ascan be seen in these figures, the DOS substantially reduces thechromatic aberration of the system. The calculated monochromatic opticaltransfer functions (not shown) for the lenses of FIGS. 1 and 2 werecomparable to those for the lens of FIG. 3.

The projection lens of Table 1 was prepared and tested. In one test, theDOS was a 16-level binary approximation kinoform prepared usingphotolithography techniques. In another test, the DOE was a blazedkinoform prepared by diamond turning. In both cases, the projectionlenses were found to work successfully except that they exhibited asomewhat lower than desired level of contrast. Some of this contrastloss is believed to be due to the fact that the kinoforms were not madeperfectly. Another source of contrast loss is believed to be the extentof the spectral range over which the lens had reduced axial color, i.e.,480 to 640 nanometers.

FIG. 6 is a schematic diagram of a CRT projection television 10constructed in accordance with the invention. As shown in this figure,projection television 10 includes cabinet 12 having projection screen 14along its front face and slanted mirror 18 along its back face. Module13 schematically illustrates a lens system constructed in accordancewith the invention and module 16 illustrates its associated CRT tube. Inpractice, three lens systems 13 and three CRT tubes 16 are used toproject red, green, and blue images onto screen 14.

Although specific embodiments of the invention have been described andillustrated, it is to be understood that a variety of modificationswhich do not depart from the scope and spirit of the invention will beevident to persons of ordinary skill in the art from the foregoingdisclosure.

TABLE 1 Surf. Clear Aperture No. Type Radius Thickness Glass Diameter 1a 55.6214 9.00000 ACRYLIC 86.77 2 a 55.9468 22.11697 74.61 3 ∞ 0.0000068.19 4 63.9494 18.00000 BACD5 67.59 5 −134.1724 1.00000 66.45 6 ∞0.00500 ****** 62.17 7 ∞ 6.00000 ACRYLIC 62.17 8 ∞ 9.48017 58.89 9 a−903.7057 8.00000 ACRYLIC 58.91 10 a −96.6566 Space 1 59.68 11 a−40.3999 4.00000 ACRYLIC 65.57 12 −42.6000 9.00000 432500 71.72 13 2814.10000 562500 130.00 14 −350.0000 Image distance 130.00 SymbolDescription a-Polynomial asphere Object and Image Surface Surface RadiusImage −350.0000 Even Polynomial Aspheres Surf. No. D E F G H I 1−1.2325E-06 −7.5049E-10 −2.7130E-13 −4.1105E-16  4.4689E-19 −1.0050E-222 −2.1192E-07  3.0589E-11 −1.9537E-13 −1.3666E-15  1.4025E-18−3.2263E-22 9  9.1273E-07  1.9563E-09  2.4017E-12 −2.9884E-15 1.2489E-18 −1.2894E-22 10  2.3025E-06  3.1721E-09 −6.1199E-12 1.9111E-14 −2.0515E-17  7.8340E-21 11 −9.3441E-06  3.6121E-08−7.8947E-11  9.0452E-14 −5.1506E-17  1.0853E-20 Variable Spaces ZoomSpace 1 Focal Image Pos. T(10) Shift Distance 1 27.225 −0.077 0.000 226.526  0.080 0.000 First-Order Data f/number 1.29 1.27 Magnification−0.1167 −0.1013 Object Height −584.28 −673.15 Object Distance −672.07−769.94 Effective Focal Length 71.134 71.629 Image Distance 0.00 0.00Overall Length 800.00 897.17 Forward Vertex Distance 127.93 127.23Barrel Length 127.93 127.23 Stop Surface Number 4 4 Distance to Stop18.39 18.39 Stop Diameter 65.166 65.205 Entrance Pupil Distance 46.27146.271 Exit Pupil Distance −54.690 −54.331 First Order Properties ofElements Element Surface Number Numbers Power f′ 1 1 2  0.52370E-031900.5 2 4 5  0.13195E-01 75.786 5 9 10  0.45771E-02 218.48 6 11 12−0.25187E-03 −3970.3 7 12 13 −0.10141E-01 −98.611 8 13 14  0.16057E-02622.78

TABLE 2 Surf. Clear Aperture No. Type Radius Thickness Glass Diameter 1a 48.2124 6.00000 ACRYLIC 86.51 2 a 48.8057 26.68512 76.70 3 62.229518.00000 BACD5 68.70 4 −170.9164 1.00000 67.35 5 ∞ 0.00500 ****** 63.816 ∞ 6.00000 ACRYLIC 63.81 7 ∞ 6.60000 60.54 8 ∞ 3.91941 58.16 9 a−1279.6190 7.00000 ACRYLIC 61.57 10 a −94.3146 Space 1 61.24 11 a−43.9745 4.00000 ACRYLIC 67.01 12 −44.0000 9.00000 432500 73.18 13 2814.10000 562500 113.98 14 −350.0000 Image distance 124.74 SymbolDescription a-Polynomial asphere Object and Image Surface Surface RadiusImage −350.0000 Even Polynomial Aspheres Surf. No. D E F G H I 1−1.3620E-06 −3.0451E-11 −1.6856E-12 −9.8743E-17  6.9042E-19 −2.0047E-222 −4.0532E-07  5.7926E-10 −1.6747E-12 −7.2393E-16  1.2554E-18−2.8790E-22 9  1.4074E-06 −6.4383E-10  8.5939E-12 −8.5760E-15 5.1244E-18 −1.6711E-21 10  2.1954E-06  4.1546E-09 −6.3741E-12 1.7081E-14 −1.4451E-17  4.3058E-21 11 −8.8062E-06  3.1881E-08−7.2816E-11  8.6836E-14 −5.1853E-17  1.1727E-20 Variable Spaces ZoomSpace 1 Focal Image Pos. T(10) Shift Distance 1 27.960 −0.361 0.001 227.243 −0.205 0.000 First-Order Data f/number 1.27 1.26 Magnification−0.1167 −0.1013 Object Height −584.20 −673.10 Object Distance −660.93−767.93 Effective Focal Length 71.548 72.003 Image Distance 0.53677E-030.43109E-03 Overall Length 800.20 897.48 Forward Vertex Distance 130.27129.55 Barrel Length 130.27 129.55 Stop Surface Number 4 4 Distance toStop −0.75 −0.75 Stop Diameter 65.234 64.831 Entrance Pupil Distance46.909 46.909 Exit Pupil Distance −57.231 −56.848 First Order Propertiesof Elements Element Surface Number Numbers Power f′ 1 1 2  0.54070E-031849.5 2 3 4  0.12592E-01 79.414 5 9 10  0.48590E-02 205.80 6 11 12−0.33091E-03 3022.0 7 12 13 −0.98182E-02 −101.85 8 13 14  0.16057E-02622.78

TABLE 3 Surf. Clear Aperture No. Type Radius Thickness Glass Diameter 1a 52.8412 9.00000 ACRYLIC 87.52 2 a 53.6487 23.92789 74.93 3 ∞ 0.0000067.51 4 62.3370 18.00000 BACD5 68.53 5 −151.5454 8.00000 67.34 6 ∞5.71170 57.52 7 a 1639.1639 9.00000 ACRYLIC 59.76 8 a −97.1935 Space 160.26 9 a −40.3999 4.00000 ACRYLIC 65.82 10 −42.6000 9.00000 43250072.01 11 ∞ 14.10000 562500 130.00 12 −350.0000 Image distance 130.00Symbol Description a-Polynomial asphere Object and Image Surface SurfaceRadius Image −350.0000 Even Polynomial Aspheres Surf. No. D E F G H I 1−8.3461E-07 −5.6730E-10 −4.5333E-13 −4.3712E-16  4.7297E-19 −1.0474E-222  4.0601E-07 −2.8649E-10  2.6176E-14 −1.3875E-15  1.2042E-18−2.3346E-22 7  6.6165E-07  0.2944E-10  3.2892E-12 −3.1069E-15 1.2133E-18 −1.0162E-23 8  1.8506E-06  2.9676E-09 −6.7252E-12 1.9458E-14 −1.9846E-17  7.6258E-21 9 −0.3441E-06  3.6121E-08−7.8947E-11  9.0452E-14 −5.1506E-17  1.0853E-20 Variable Spaces ZoomSpace 1 Focal Image Pos. T(8) Shift Distance 1 27.305 −0.088 0.000 226.612  0.080 0.000 First-Order Data f/number 1.29 1.25 Magnification−0.1167 −0.1013 Object Height −584.28 −673.15 Object Distance −671.98−770.12 Effective Focal Length 71.364 71.857 Image Distance 0.00−10587E-03 Overall Length 800.02 897.47 Forward Vertex Distance 128.04127.35 Barrel Length 128.04 127.35 Stop Surface Number 4 4 Distance toStop 18.61 18.61 Stop Diameter 64.262 66.019 Entrance Pupil Distance49.132 40.132 Exit Pupil Distance −54.667 −54.311 First Order Propertiesof Elements Element Surface Number Numbers Power f′ 1 1 2  0.65884E-031517.8 2 4 5  0.12971E-01 77.093 3 7 8  0.53724E-02 186.14 4 9 10−0.25187E-03 −3970.3 5 10 11 −0.10141E-01 −98.611 6 11 12  0.16057E-02622.78

TABLE 4 FIG. 4A FIG 4B FIG. 4C Focal length 71.13 71.55 71.36Magnification −0.117 −0.117 −0.117 f/ number 1.29 1.27 1.29 Image height−584.28 −584.20 −584.28 Object height 61.64 62.30 61.90

TABLE 5 FIG. 5A FIG. 5B FIG. 5C Focal length 71.63 72.00 71.86Magnification −0.101 −0.101 −0.101 f/ number 1.27 1.26 1.25 Image height−673.15 −673.10 −673.15 Object height 61.50 62.18 61.81

TABLE 6 Ex. No. f0 f1 f2 fcr f3 f_(DOE) 1/2 w 1 71.13 1909.5 75.79218.48 −113.60 2000.0 40.5° 2 71.55 1849.5 79.41 205.80 −126.78 2000.040.2°

What is claimed is:
 1. A projection lens system for use with a cathoderay tube, said projection lens system having a long conjugate side and ashort conjugate side and comprising in order from its long conjugateside: (a) a first lens unit which primarily corrects aperture dependentaberrations, said first lens unit having a short conjugate side andcomprising at least one aspherical surface; (b) a second lens unithaving a positive optical power; (c) a corrector lens unit comprising atleast one aspherical surface; and (d) a third lens unit which isassociated with the cathode ray tube during use of the lens system andwhich provides correction for the field curvature of the lens system,said third lens unit having a long conjugate side; wherein the lenssystem includes at least one diffractive optical surface which at leastpartially corrects the axial color of the lens system and which islocated between the short conjugate side of the first lens unit and thelong conjugate side of the third lens unit.
 2. The projection lenssystem of claim 1 wherein the diffractive optical surface is formed on adiffractive optical element which comprises two optical surfaces, one ofsaid optical surfaces being plano and the other of said optical surfacesbeing the diffractive optical surface.
 3. The projection lens system ofclaim 1 wherein: (i) the projection lens system has an aperture stop anda focal length f₀, and (ii) the distance between the diffractive opticalsurface and the aperture stop is less than 0.1·f₀.
 4. The projectionlens system of claim 3 wherein the distance between the diffractiveoptical surface and the aperture stop is less than 0.05·f₀.
 5. Aprojection television system comprising a cathode ray tube and aprojection lens system for projecting light from the cathode ray tubeonto a screen to form an image, said projection lens system comprisingthe projection lens system of claim
 1. 6. A projection television systemcomprising three cathode ray tubes and three projection lens systems,one projection lens system being associated with each of the cathode raytubes for projecting light from that tube onto a common screen to forman image, each projection lens system comprising the projection lenssystem of claim 1.